This invention relates generally to optical distortion analyzing systems, and, more particularly to an optical distortion analyzer system which is capable of automatically quantifying with high accuracy and repeatability the optical quality of large area transparencies such as, for example, those found in aircraft windscreens, ground vehicle windshields, windows or the like.
It is often necessary to measure and evaluate the optical quality of large area transparencies used in aircraft, ground transporters, and other windows. The optical quality refers to the manner in which the transparency transmits images without distortion in terms of size, orientation, displacement, deviation and aspect angle of the image. This evaluation is performed for the purposes of quality control in fabrication, selection for use in critical applications, and for feedback into a corrective process such as reforming, polishing, etc.
Current systems utilized in the testing of aircraft transparencies and the like generally do not permit sufficient accuracy to quantify optical imperfections that can be detected by the human eye. Such prior art testing systems usually require photographing a grid pattern through the windscreen or transparency. From the grid photographs, distortion is quantitated as a slope change in the otherwise straight lines of the photographed image. An analysis as presented hereinbelow clearly illustrates the significant limitations of such prior art systems.
More specifically, the camera and windscreen or transparency are treated as a compound lens system where f.sub.c =the camera focal length, f.sub.2 =the local lens power of the distortion of the windscreen, r=their separation distance, f=the effective system focal length, p=the image distance, g=the object distance, x=the image size, and y=the object size. Magnification M, is given by the expression EQU M=f/(p-f). (1)
Differentiating Equation 1, one obtains the fractional change in magnification given by a fractional change in focal length, f, i.e., EQU dM=(p/(p-f).sup.2)df (2)
Let df=.DELTA.f=f.sub.c -f.sub.r, where f.sub.r =the focal length of the system with the presence of local distortion anomalie, f.sub.w, i.e., EQU f.sub.r =f.sub.c f.sub.w /(f.sub.c +f.sub.2 -r) (3)
Now x=My, where M is the magnification, also EQU dx/x=(1/M)dM (4)
where dx/x=the fractional change in image size. Substituting into Equation 4 the expression for dM of Equation 2, and eliminating df, one obtains an expression for the image size change as a function o the test configuration parameters: ##EQU1## A typical numerical example would have g=4.57 m, p=0.09 m, r=1.0 m, y=0.025 m, f.sub.c =0.05 m. The human eye can detect imperfections as low as 0.05 diopters in a transparency translated with respect to objects sighted at infinity. Assuming the local distortion on the transparency has a lens effect of 0.05 diopters or f.sub.w =l/0.05=20 m, from Equation 5, dx=14 .mu.m. This small spatial change approaches the grain size of some film. Therefore, this level of distortion cannot be conveniently evaluated by such prior art procedures.
A need therefore exists to provide an analyzer system which is capable of accurately and reliably quantifying the optical quality of transparencies. In addition, it would be highly desirable for this sytem to be produced economically.